<!DOCTYPE html>
<html lang="zh-CN">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .outer {
            width: 1000px;
            height: 100px;
            border: 1px solid black;
        }

        .outer:hover .box {
            width: 1000px;
        }

        .box {
            width: 100px;
            height: 100px;
            background-color: orange;
            transition: 3s all .5s linear;
        }
    </style>
</head>

<body>
    <div class="outer">
        <div class="box"></div>
    </div>
</body>

</html>